1. Field of the Invention
The present invention relates to a surface light-emitting device and a liquid crystal display device.
2. Description of the Related Art
Hitherto, in reflective liquid crystal display devices using ambient light as a light source, the brightness depends on the amount of ambient light, and thus the display visibility significantly deteriorates in circumstances where a sufficient amount of ambient light cannot be obtained, such as in dark places.
In order to overcome this problem, a liquid crystal display device in which a front light (surface light-emitting device), functioning as an auxiliary light source, is provided on the front surface of a reflective liquid crystal display unit has been proposed. This type of liquid crystal display device including the front light operates as an ordinary reflective liquid crystal display device in circumstances where sufficient ambient light can be obtained, such as outdoors during daytime. Also, the front light can be used as a light source as required. FIG. 12 is a sectional view showing an example of a liquid crystal display device provided with a front light on the front surface of a liquid crystal display unit. The liquid crystal display device 200 shown in FIG. 12 includes a liquid crystal display unit 220 and a front light 210. The front light 210 is placed on the front surface (upper surface in FIG. 12) of the liquid crystal display unit 220 so that a light guide panel 212 is placed in the display area of the liquid crystal display unit 220.
The front light 210 includes the light guide panel 212, which is made by injection molding of a transparent acrylic resin or the like, and a light source 213 comprising a cold-cathode tube or the like provided on an end surface 212a of the light guide panel 212. The lower surface (the liquid crystal display unit 220 side) of the light guide panel 212 is an exit surface 212b from which light is emitted. The surface facing the exit surface 212b (the upper surface of the light guide panel 212) is a reflecting surface 212c provided with a first slope 214a formed to incline with respect to the exit surface 212b so as to change the direction of light inside the light guide panel 212 and a second slope 214b which is continuous with the first slope 214a, the first and second slopes 214a and 214b being repeatedly placed one after another.
The liquid crystal display unit 220 includes a first substrate 221 and a second substrate 222, which face each other with a liquid crystal layer 223 therebetween, and a sealing material 224 for joining the first substrate 221 and the second substrate 222. On the liquid crystal layer 223 side of the first substrate 221, a reflecting layer 230 including an organic film 228 and a reflecting film 225, which is formed on the organic film 228 and which reflects incoming light, and a display circuit 226 for driving and controlling the liquid crystal layer 223 are laminated in this order. Also, a display circuit 227 is formed on the liquid crystal layer 223 side of the second substrate 222. As shown in FIG. 12, the upper surface (the surface on the liquid crystal layer 223 side) of the organic film 228 is uneven so that the light reflected by the reflecting film 225 formed on the surface of the organic film 228 is diffused.
In the liquid crystal display device 200 having the above-described configuration, light from the light source 213 enters the light guide panel 212 via the end surface 212a of the light guide panel 212, passes through the light guide panel 212, is reflected at the first slope 214a having the larger slope angle with respect to the light guide direction, the direction of the light is changed toward the exit surface 212b, and the light is radiated from the exit surface 212b. The light from the exit surface 212b enters the liquid crystal display unit 220 as illuminating light, passes through the display circuits 226 and 227 and the liquid crystal layer 223, is reflected by the reflecting film 225, returns to the outside of the liquid crystal display unit 220, passes through the exit surface 212b and the reflecting surface 212c of the light guide panel 212, and reaches the user. In this way, the display of the liquid crystal display unit 220 is seen by the user.
The liquid crystal display device 200 having the above-described configuration can be used even in dark places, where ambient light cannot be used, by turning on the front light 210 as required. However, when display brightness using ambient light as a light source is compared to the display brightness using the front light 210, the display brightness using the front light 210 is lower. Accordingly, when display is performed by switching on/off the front light, the display is difficult to see due to the difference in brightness, and therefore the device is not user-friendly.
The present invention has been made to solve the above-described problems, and it is an object of the present invention to provide a surface light-emitting device which has a high luminance and a good brightness distribution in the display, and which is preferably provided on the front surface of a reflective liquid crystal display unit.
Also, it is another object of the present invention to provide a liquid crystal display device having a surface light-emitting device in which a bright display can be realized when the surface light-emitting device is either on or off and an excellent visibility can be realized.
The above-described reflective liquid crystal display device 200 including the front light 210 is used for a display unit of mobile phones and mobile information terminals. The liquid crystal display device incorporated into such electronic equipment is often used so that the direction of an external light source and the direction of the line of sight of a user receiving the reflection light from the liquid crystal display device 200 are in a specific range. This will be described below with reference to FIGS. 12 and 13.
FIG. 13 is a side view showing an example of a mobile phone including the above-described liquid crystal display device 200 as a display unit. The mobile phone 250 shown in this figure includes an operation unit 260 and a display unit 270. The operation unit 260 and the display unit 270 are coupled to each other by a hinge 280 provided therebetween and the display unit 270 can be opened and closed with respect to the operation unit 260, with the hinge 280 being a supporting point. A plurality of operation buttons 261 are provided on the inner side of the operation unit 260 and the liquid crystal display device 200 is provided on the inner side of the display unit 270. When used, the display unit 270 is opened, with the hinge 280 acting as a supporting point, as shown in FIG. 13, so that the liquid crystal display device 200 provided on the inner side of the display unit 270 can be seen by the user.
The liquid crystal display device 200 provided in the mobile phone 250 shown in FIG. 13 performs display by allowing the reflecting layer 230 of the liquid crystal display unit 220 to reflect incoming light Q entering at a predetermined angle with respect to the normal line P of the liquid crystal display device 200, when the external light is used as a light source. At that time, the user U is often located a little away from the normal line P of the liquid crystal display device 200 (the operation unit 260 side) as shown in FIG. 13. Accordingly, the inventors of the present invention have found that the direction of reflection light Qxe2x80x2 generated by specularly reflecting the incoming light Q should substantially correspond with the direction of the line of sight of the user U in order to make the display brighter in the direction of the line of sight of the user U.
The inventors have studied the luminance characteristic of a surface light-emitting device for improving the visibility of the liquid crystal display device based on the above-described knowledge and have completed the present invention.
That is, according to an aspect of the present invention, a surface light-emitting device comprises a light source; and a light guide panel having a light incident surface provided on an end surface thereof that introduces light from the light source and an exit surface from which the light is radiated. The direction in which the amount of light radiated from the exit surface of the light guide panel is maximized defines an angle of 1xc2x0 to 10xc2x0 with respect to the normal line of the exit surface.
With this configuration, when the surface light-emitting device of the present invention is provided on the front surface of a reflective liquid crystal display unit so as to constitute a liquid crystal display device, the amount of reflection light in the direction of the line of sight of a user can be maximized. That is, the light from the surface light-emitting device is radiated at an angle of 1xc2x0 to 10xc2x0 with respect to the normal line of the exit surface, and thus the incident angle of light entering the reflector provided in the liquid crystal display unit is also 1xc2x0 to 10xc2x0. Accordingly, the angle of specular reflection light with respect to the incoming light is also 1xc2x0 to 10xc2x0, and the amount of the light radiated from the liquid crystal display unit is maximized in the direction defining an angle of 1xc2x0 to 10xc2x0 with respect to the direction perpendicular to the liquid crystal display unit. Since this direction substantially corresponds to the direction of the line of sight of the user, the brightness of the liquid crystal display device is maximized in the direction of the line of sight of the user, and thus a bright display can be achieved.
Also, by enhancing the brightness when the surface light-emitting device is in the on state, the difference in the brightness in a state where display is performed by using external light (when the surface light-emitting device is in the off state) becomes small. Accordingly, the surface light-emitting device can be preferably used by switching it on/off.
Preferably, the light guide panel has the luminance characteristic in that the luminance is maximized in the direction defining an angle of 3xc2x0 to 8xc2x0 with respect to the normal line of the exit surface.
With this arrangement, in the liquid crystal display device including the surface light-emitting device of the present invention as an illuminating unit, the direction of reflection light can be brought closer to the direction of the line of sight of the user, and thus a brighter display can be obtained.
According to another aspect of the present invention, a surface light-emitting device comprises a light source; and a light guide panel having a light incident surface provided on an end surface thereof for introducing light from the light source and an exit surface from which the light is radiated. The light guide panel comprises a reflecting surface which faces the exit surface and which is provided with a plurality of grooves formed sequentially in a stripe pattern, each of the grooves having a gentle slope and a steep slope whose slope angle is steeper than that of the gentle slope. The slope angle of the gentle slope is 1.8xc2x0 to 2.5xc2x0, the pitch of the grooves is 140 xcexcm to 240 xcexcm, and the slope angle of the steep slope is 43xc2x0 to 47xc2x0.
By setting the slope angle of the steep slope to 43xc2x0 to 47xc2x0, the direction in which the amount of light from the surface light-emitting device is maximized can be set in the range of 1xc2x0 to 10xc2x0 with respect to the normal line of the exit surface, and thus the illuminated liquid crystal display unit can efficiently reflect the light. Therefore, a liquid crystal display device which performs a bright display and which is excellent in visibility can be provided. When the slope angle of the steep slope is less than 43xc2x0, the angle defined by the direction in which the luminance of the surface light-emitting device is maximized and the normal line of the exit surface becomes less than 1xc2x0, and thus an effect for realizing a bright display cannot be obtained. Also, when the slope angle of the steep slope is more than 47xc2x0, the angle defined by the direction in which the amount of the radiated light is maximized and the normal line of the exit surface becomes too large, and thus a bright display cannot be realized.
The slope angle of the gentle slope of the light guide panel is 1.8xc2x0 to 2.5xc2x0. If the slope angle of the gentle slot is less than 1.8xc2x0, the amount of light from the surface light-emitting device is reduced and the brightness of the liquid crystal display device is reduced accordingly. Also, when the slope angle is more than 2.5xc2x0, the distribution of radiated light over the exit surface of the surface light-emitting device becomes nonuniform, and thus the brightness distribution of the liquid crystal display device is deteriorated, which is not preferable.
The pitch of the groove of the light guide panel is 140 xcexcm to 240 xcexcm. If the pitch of the groove is less than 140 xcexcm, the amount of light radiated from the exit surface of the surface light-emitting device is reduced, and the brightness of the liquid crystal display device is reduced accordingly. Also, when the pitch of the groove is more than 240 xcexcm, emission lines are generated in the light guide panel of the surface light-emitting device and the visibility of the liquid crystal display device is disadvantageously deteriorated.
According to another aspect of the present invention, a liquid crystal display device comprises the above-described surface light-emitting device. With this arrangement, the amount of radiated light in the direction of the line of sight of the user can be increased by the surface light-emitting device having the above-described characteristic, and thus the liquid crystal display device for performing a bright display can be realized.
The liquid crystal display device may further comprise a liquid crystal display unit including a pair of substrates sandwiching a liquid crystal layer. The surface light-emitting device is provided on the front surface of the liquid crystal display unit. The liquid crystal display unit includes a reflector that reflects light from the outer side of one of the substrates. The direction in which the amount of reflection light reflected by the reflector is maximized defines an angle of 1xc2x0 to 10xc2x0 with respect to the normal line of the liquid crystal display unit when the surface light-emitting device is in the on state.
Preferably, the direction in which the amount of reflection light reflected by the reflector is maximized defines an angle of 3xc2x0 to 8xc2x0 with respect to the normal line of the liquid crystal display unit when the surface light-emitting device is in the on state.
With this configuration, the liquid crystal display device in which the brightness in the direction of the line of sight of the user using the liquid crystal display device is enhanced can be realized, and the liquid crystal display device for performing a bright display can be realized.
The reflector may have a surface provided with a plurality of reflective concavities formed irregularly and sequentially. With this arrangement, the reflection efficiency of the reflector can be increased and thus the liquid crystal display device for performing a brighter display can be realized.
The reflector may be provided on the back side of the liquid crystal display unit. Accordingly, a liquid crystal display unit of an arbitrary form and the reflector can be combined.
Further, the reflector may be incorporated into the liquid crystal display unit. With this configuration, the path of the light entered the liquid crystal display unit and reaching the reflector can be shortened, and thus a bright display can be realized while suppressing a loss of light.
Note that in the present specification, the numerical values presented, unless otherwise specified as exactly those values, are approximate, e.g. xe2x80x9c140 xcexcmxe2x80x9d is xe2x80x9cabout 140 xcexcmxe2x80x9d unless specified as xe2x80x9cexactly 140 xcexcmxe2x80x9d. Similarly, each range is indicated as xe2x80x9cx to y.xe2x80x9d Such a range represents xe2x80x9cabout x to about yxe2x80x9d unless otherwise designated as exactly using the particular values. For example, the range of 140 xcexcm to 240 xcexcm is about 140 xcexcm to 240 xcexcm. Additionally, from time to time xe2x80x9caboutxe2x80x9d may also be used as prefixes to values or ranges, but are merely present to remind the reader that these numbers are approximate as long as the desired characteristics are obtained.